German utility model 84 35 585 describes a sieve with bar-shaped beaters that are secured to an elastic mat that is mounted underneath the flexible screen. Both screens are mounted on tensioning rods that reciprocate and thus insure that the lower screen is taut and strikes with its beaters against the lower face of the upper screen in order to knock big particles out of the upper screen. The provision of a second screen underneath the first one is expensive and entails considerable extra manufacturing time.